Microwave coupling devices ("couplers") are used to join two waveguide structures through which one or more microwave signals propagate. In a typical microwave coupler application, the coupler may be used to link two waveguide structures having different propagation modes. In a more specific coupler application, a combiner-type coupler is often used to "feed" an antenna from a waveguide structure such that the antenna transmits or receives signals in two or more frequency bands. In each instance, the microwave coupler would be designed to provide the appropriate waveguide transition between the respective structures. An improper transition in such microwave couplers can cause an unacceptable VSWR and typically results in significant signal distortion. Signal distortion introduces the propagation of signals in a multitude of undesired higher order modes, often referred to as "overmoding." Such "overmoding" adversely affects both the bandwidth and the quality of the propagating signals.
In the prior art, the magnitude of such higher order modes has been lessened by careful dimensioning of the waveguide to provide a cut-off point beyond which these modes will not operate. Unfortunately, such dimensioning by itself does not accommodate many applications in which the combiner or coupler propagates signals in more than one frequency band.
There are previously known combiner structures that propagate signals in two frequency bands, However, they require costly or elaborate combiner structures to transform the propagation modes from the respective waveguide paths into a common path operating in a signal propagation mode. For example, one such structure includes a tuning choke which is used as part of a dual band junction in which signals from two frequency bands are respectively passed into the outer and inner conductors of a coaxial waveguide. Another type employs a conically shaped cone having a circular waveguide coupled at its base through which a signal from one frequency band passes, and has four openings through its side wall through which a signal from one frequency band, represented by two orthogonal polarizations, passes. The orthogonal polarizations which pass through the side wall are fed respectively from separate hybrid tees with electrically balanced waveguide connecting structures. These structures are not only costly to build, but the two bands that they accommodate are relatively narrow and, therefore, are limited in their signal carrying capacity. Attempts to expand that capacity have resulted in intolerable signal distortion.
Accordingly, there is a need for a coupling structure that overcomes the aforementioned deficiencies.